Corrosion-resistant uranium



United States Patent 3,547,709 CORROSION-RESISTANT URANIUM George S.Petit and Ralph R. Wright, Oak Ridge, Tenn.,

assignors to the United States of America as represented by the UnitedStates Atomic Energy Commission No Drawing. Filed May 14, 1968, Ser. No.728,910 Int. Cl. C23f 7/02 US. Cl. 148-63 10 Claims ABSTRACT OF THEDISCLOSURE Metallic uranium is provided with a surface film of uraniumoxide which is highly protective against corrosion-producing agents suchas dry or moist air and water. The corrosion-resistant uranium oxidefilm is provided by heating the uranium in an evacuated furnace to atemperature in a range of about 500 to 650 C. and then contacting theuranium with a selected quantity of dry oxygen.

The present invention relates generally to corrosionresistant uraniumand more particularly to a method of providing metallic uranium with acorrosion-resistant film of uranium oxide. This invention was made inthe course of, or under, a contract with the US. Atomic EnergyCommission.

Metallic uranium is by nature a highly reactive metal which readilyreacts with and is corroded by many gases and liquids including air andwater. Extensive investigations have been previously conducted in aneffort to minimize or obviate this corrosion problem. These efforts haveresulted in several techniques for alleviating uranium corrosion andinclude such protective measures as alloying, plastic coatings, coveringexposed surfaces with layers of nickel or aluminum by vapor deposition,electroplating with nickel, etc.

While previous techniques for protecting uranium metal from corrosionsuch as those mentioned above have met with some success, the practiceof these techniques is often very difficult and cumbersome with theresulting corrosion barrier in certain instances failing to accomplishits intended purpose. For example, the tendency of uranium metalsurfaces to become passive after preplating treatment renders uranium ahighly difficult metal to successfully plate. Further, imperviousprotective platings are not easily provided, particularly when theplatings are relatively thin, i.e., of a thickness in the order of about1 mil (0.001 of an inch).

It is the aim of the present invention to provide exposed surfaces ofmetallic uranium with a protective layer or film of uranium oxide whichis very adherent, hard, and sufliciently resistant to penetration by orreaction with air and water to prevent corrosion of the uranium evenwhen exposed to severe corrosion-causing conditions such as prolongedexposure to heated air containing water vapor. The novel protectiveuranium oxide film afforded by the present invention is formed ofuranium oxides which are not wettable by water or reactive with moist ordry air and water. These uranium oxides consist of a gradient of uraniumdioxide and lesser oxides including uranium monoxide with the uraniumdioxide being at the surface of the protective film and the uraniummonoxide being at the interface between the metallic uranium and thefilm.

An object of the present invention is to provide metallic uranium with asurface film or coating which obviates or substantially minimizescorrosion of the uranium.

Another object of the present invention is to provide metallic uraniumwith a surface film of uranium oxide which is resistant to penetrationor reaction with air and Water vapor.

A further object of the present invention is to provide a method oftreating metallic uranium in order to render the latter resistant tocorrosion by air and water. The method comprises the formation of auranium oxide film on the surface of the metallic uranium by heating theuranium in an evacuated chamber and contacting the uranium with aselected quantity of dry oxygen.

Other and further objects of the invention will be obvious upon anunderstanding of the illustrative method for protecting uranium metalfrom corrosion about to be described, or will be indicated in theappended claims, and various advantages not referred to herein willoccur to one skilled in the art upon employment of the invention inpractice.

Described generally, the present invention relates to the treatment ofuranium metal for rendering it highly resistant to attacks by normallycorrosive agents, particularly water and moist or dry air. The uraniummetal is protected from these corrosive agents by providing the uraniummetal with a surface film of uranium oxide which is composed of uraniumdioxide (U0 and lesser oxides in a gradient ranging down to uraniummonoxide (UO). This film is produced on. the metallic uranium by heatingthe uranium metal in vacuum after the surfaces thereof have beencleansed of excess naturally occurring oxides and other contaminants andthereafter contacting the heated uranium With a selected quantity ofvirtually moisture-free or dry oxygen.

The protective film of the present invention is believed to be formed bya mobile layer of U0. A thin film of U0 will form on the uranium metalin the furnace due to the residual oxygen in the furnace atmosphere andthe oxygen on the surface of the uranium metal. With the uranium metalinthe evacuated furnace a preselected quantity of oxygen is slowlyadmitted into the furnace and comes in contact with the metallicuranium. This contact between the oxygen and the thin UO film on themetallic uranium causes the U0 to consume the oxygen and form a thinlayer of U0 which, in the presence of excess uranium metal, will permitthe reaction for forming a new layer of U0. Thus, the interface betweenthe uranium oxide film and the metal continually progresses into theuranium metal while at the same time maintaining the same chemicalbonding as the initial film (U0) formed. There is probably somediifusion of oxygen through the uranium oxide film, permitting theformation of some UO due to a limited quantity of oxygen reactingdirectly with excess uranium metal at the interface. As the quantity ofexcess uranium diminishes with the increasing thickness of theprotective film, the quantityof oxygen in the film increases. Thus,there is a gradually increasing gradient of oxygen content in theprotective film from the interface surface outwardly. The structure ofthe film effected by this gradient ranges from U0 at the interface to U0at the film surface, with the latter providing the predominantproportion of the protective film thickness. An analytical determinationperformed on the protective film indicated that the oxygento-uraniumratio of the film is about 1.9. Accordingly, in a film thickness ofabout 0.2 to 1.0 mil (0.001 of an inch), the surface of the uraniummetal is protected by U0 rather than U0.

The use of excess quantities of dry oxygen for the formation of auranium oxide film on metallic uranium can be deleterious in that thefilm resulting from the exposure of the heated uranium to excessiveamounts of oxygen is a relatively thick, non-wetting layer of U0 whichis not significantly protective and which often exhibits cracking andflaking. It is believed that this nonprotective film results when asufficient quantity of dry oxygen is present to cause the directformation of U by the reaction U+O; UO rather than the indirectformation indicative by the reaction described above.

In order to provide the corrosion-resistant uranium oxide film of thepresent invention, the uranium metal to be treated must be thoroughlycleansed prior to the for mation of the protective film thereon. Thenaturally occurring uranium oxide on the surface of the metallic uraniummust be removed prior to heating the metallic uranium in the presence ofdry oxygen since this oxide inhibits the formation of the protectivefilm. The layer of natural uranium oxide may be readily removed byimmersing the uranium in an acid bath, such as, for example, a solutionof 8 N nitric acid, for approximately 20 minutes. Other contaminants maybe removed prior to the acid bath by using conventional cleansingprocedures such as scouring powders and distilled water rinses.

The best results provided by the protective uranium oxide film areobtained on metallic uranium articles having a surface finish of 80 rootmeans square (RMS) or better. Pits and irregularities in the uraniummetal produce the weakest areas in the film and corrosion will normallyoccur in these areas first, if at all. However, even when corrosionoccurs in these relatively small areas, it does not spread over theentire piece as would normally occur with uranium metal which has beensubjected to corrosion inhibiting techniques as previously practiced.

The heating of the metallic uranium for producing a protective film maybe readily accomplished in an electrically heated vacuum furnace ofconventional design that is capable of being fitted with oxygen leaks.This furnace should also be capable of being evacuated to a pressure ina range corresponding to about 1 to 10 microns and have a leak rate lessthan about one micron per hour.

After cleaning the metallic uranium article as above described, or inany other suitable manner, the uranium article is immediately placed inthe furnace, which is then sealed and evacuated to a pressure within thedesired range without delay. During this pumping down, it may bedesirable to assure an oxygen-free atmosphere within the furnace bypurging the latter with a suitable inert gas such as argon or the like.When the furnace is heated to a temperature sufficient to effect theformation of the protective film, a selected quantity of dry oxygen isintroduced into the furnace. Normally, the temperatures sufiicient toprovide the desired film are in the range of about 500 to about 650 C.It has been found that the better results occur at the highertemperatures, but care should be exercised to assure that thetemperature does not get up to or above 665 C. since uranium changesfrom the alpha to the beta phase at about this temperature. It isbelieved that the conversion of the uranium metal to the beta phaseshould be avoided since the subsequent return to the alpha phase uponcooling would be accompanied by a crystal growth which might causewarpage and disruption of the protective film. The rate of heatup of theuranium article in the furnace is not particularly critical. However,the heatup rate should be sufficiently slow to permit outgassing ofhydrogen and other entrapped gases from the uranium prior to theintroduction of the dry oxygen into the furnace.

Upon obtaining a preferred furnace temperature of about 625 C., asuitable quantity of dry oxygen is introduced into the furnace in anysuitable manner such as by providing the furnace with a number ofstandard leaks connected in parallel, with each leak being capable ofproviding the furnace volume with a particular quantity of oxygen.Satisfactory results have been achieved by using three leaks, eachcapable of providing the furnace interior with 900 microns of oxygen percubic foot per hour when treating uranium articles having a surface areaof about 7 square inches. It has been found that about 900 microns ofoxygen per cubic foot per hour in the furnace at 625 C. will provide aprotective film of about 0.25 mil thickness in one hour on a uraniumarticle having about 7 square inches of surface area. With uraniumarticles of surface areas greater than about 7 square inches,correspondingly greater quanties of oxygen are required to provideprotective films in the desired thickness range. In other words, with auranium article having a surface area of about 14 square inches, thefurnace interior should be provided with about 1800 microns of oxygenper cubic foot per hour to provide a protective film with a thickness ofabout 0.25 mil. The type of oxygen used in the furnace is highlycritical in that it is necessarily free, or virtually free, of moisturesince the protective film cannot be obtained when excessive moisture ispresent. The use of commercially available 99.9% pure oxygen has beenfound to be adequately dry for accomplishing the method of the presentinvention. The presence of an inert gas, e.g., argon, in the furnacedoes not appear to have any deleterious effects so long as it isvirtually or entirely moisture-free and does not interfere with thecontacting of the uranium article with the desired quantity of oxygen.

Successful preparation of the protective film of the present inventionis highly dependent upon the use of dry oxygen together with aleak-proof vacuum furnace since inleakage of atmospheric gases,particularly moisture-laden gases, into the furnace during treatmentconsiderably changes the character of the protective film and corrosionwill occur in a matter of a few hours. For example, a defective gasketof the furnace assembly permitted trace amounts of atmospheric air toleak into the furnace, which resulted in the films being of the wettable type which failed in a short time upon exposure to heated aircontaining water vapor.

As briefly mentioned above, the quantity of dry oxygen in the furnace iscritical in that the required reaction depends upon contacting theuranium metal with less than the quantity of oxygen which would causethe aforementioned undesirable reaction of U+Og UO The film thickness isvaried by the quantity of dry oxygen in the furnace as well as theduration which the uranium metal remains in the heated furnace. Filmthicknesses in the range of about 0.2 to 1.0 mil have been found to behighly satisfactory as corrosion inhibitors.

Upon completing the treatment of the uranium metal in the furnace, whichis usually accomplished in a duration of about one hour at 625 C., thefurnace is cooled down under vacuum to assure that the film is properlyestablished prior to exposing it to corrosion-causing conditions.

The uranium oxide film produced by the method of the present inventionprovides the uranium metal with highly corrosion-resistant coatings orfilms in that uranium metal coupons so treated have been exposed toatmospheric air at relative humidity and heated to 200 F. for durationsof over several hundred hours without any sign of corrosion or otheradverse eifects. The average life of the protective film is expected tobe somewhat greater than 600 hours under the severe test conditions justdescribed. In fact, treated coupons have been exposed to theseconditions for durations greater than 1700 hours without showing anycorrosion. An electron microscope evaluation of coupons after beingsubjected to the corrosion test indicated no evidence of any destructionof the film from the exposure or any damage to the uranium. Otherproperties or characteristics of the protective film include a hardnessof about 60-65 on the Rockwell C scale as compared to 2025 on the samescale for the uranium metal, a bluish color, a surface finish as good asthe finish on the uranium metal prior to treatment in the furnace, and areproducibility of film thickness better than :01 mil.

The corrosion resistance of the protective film produced by the presentmethod is apparently achieved because, unlike naturally occurringuranium dioxide, it is not wetted by water. A quick check to determineif a uranium article is adequately protected is to dip the article in acontainer of distilled water and, if the article sheds water uponremoval from the latter, it will be corrosion resistant. However, if thecoated uranium article is wetted by the water, as was the case with theuranium articles where the furnace was inadequately sealed due to agasket failure, the film will deteriorate in a relatively short timewhen subjected to the test conditions mentioned above.

While it is not entirely clear as to what particular mechanism providesthe non-wettability of the film and hence the corrosion protection, itis believed that it may be due to a particular crystal orientation ofthe U in the film. X-ray diffraction patterns indicate a highlypreferred 110 crystal orientation in a plane parallel to the surface ofthe uranium article. The U0, on the other hand, has a crystalorientation in the 100 plane.

In order to provide a more facile understanding of the invention, anexample of a typical operation for providing a uranium metal coupon withthe protective film of the present invention is set forth below. Thisexample is merely illustrative of the subject invention and is not to beconsidered in a limiting sense since the scope of the invention islimited only by the scope of the appended claims.

EXAMPLE Two uranium metal coupons (1%" x 1" x /s) were vapor decreased,scrubbed with scouring powder, rinsed in distilled water, bathed in 8 Nnitric acid for a period of 20 minutes, rinsed in distilled water, andthen dried with acetone. Immediately after this drying, the coupons wereplaced in a vacuum furnace of the type described above. The furnace wasimmediately sealed and evacuated, purged three times with argon, andpumped down to a pressure corresponding to microns. The furnace heaterswere then energized to heat the uranium coupons to a temperature of 625C. at a rate of 250 C. per hour. During this heatup, outgasing,particularly hydrogen, occurred. When the furnace reached thetemperature of 625 C., dry, 99.9 percent pure oxygen was admitted intothe furnace interior through one of the standard leaks at a rate of 900microns per cubic foot per hour. The dry oxygen entering the furnacevolume through this leak flowed across the exposed faces or surfaces ofthe uranium coupons for a duration of one hour for producing theprotective film on the uranium coupons by the above-described reactionU+=UO 2UO. During this period the furnace temperature was maintained atapproximately 625 C. Upon completing the one-hour tr ating period, thefurnace heaters were deenergized and the furnace permitted to cool undervacuum. The treated uranium metal coupons, which were coated with a bluefilm about 0.25 mil thick, were subjected to the corrosion testdescribed above, and after 378 hours no visible deterioration wasevident.

The results in the example just described have been confirmed bynumerous equivalent experiments. In addition, experiments have beenconducted in which some of the treatment parameters have been modifiedor chang d; for example, uranium coupons prepared for facilitating thereception of the coating as described above are treated in the vacuumfurnace in the absence of an oxygen bleed or in the presence of anotheroxygen source such as urano-uranic oxide having an excess of oxygen U,o+o). The urano-uranic oxide served as a controlled source of oxygen. Thefollowing table compares the results obtained when identical uraniumcoupons were heated in the vacuum furnace (a) with no oxygenintentionally added to the furnace atmosphere, (b) in the presence ofdry oxygen liberated from the urano-uranic oxide, and (c) in thepresence of dry oyxgen introduced through one or more of the standardleaks. The test bath referred to in the table is air heated to 200 F.with a moisture content corresponding to 100 percent relative humidity.

TABLE.-.A. BRIEF SUMMARY OF CORROSION TEST RESULT$ Average life inbefore showing Mode of formation, 1 hr. corrosion at 625 0. spots, hr.Comments U 0 as oxygen source Standard leak(s), Oz gas 600 In view ofthe data in the above table, it is apparent that a long-lived protectivefilm is not obtainable in the absence of oxygen. In fact, it is believedthat the shortlived film produced by this technique was formed by theeffect of the heat treatment of uranium oxide formed on the uraniumcoupon during the time it took to transfer the pretreated uranium couponfrom the cleaning and drying solutions to the furnace and/or traceamounts of oxygen in the furnace. The protective coatings as prepared byusing the oxygen bleeds are superior to those prepared with theurano-uranic oxide since the use of the controlled oxygen bleedsfacilitates greater control over the film thicknesses, while the excessoxygen in the urano-uranic oxide can vary from batch to batch. Anotherimportant advantage of the oxygen bleeds over the use of theurano-uranic oxide is the control over the time at which the oxygen ismost advantageously introduced into the furnace. It has been establishedthat the most desirable time for the introduction of the oxygen is whenthe furnace has reached the desired equilibrium temperature, preferably625 C. To this end it is believed that, when using the urano-uranicoxide form of oxygen, at least some of the available oxygen is consumedat lower temperatures and consequently cannot provide the desiredprotective film.

It will be seen that the present invention provides a unique protectivefilm for metallic uranium which provides protection against corrosion inmoist atmospheres, which significantly exceeds corrosion protectionprovided by previously practiced methods including platings. In fact,the present invention has an advantage over plating in that there is noappreciable removal of surface uranium (only a thin layer of the naturaloxide) in the preparation, whereas the plating operation normallyrequires removal of about 0.7 mil from each side of the uranium articleduring the pickling step utilized for preparing the uranium forreception of the plating. With the present invention it is onlynecessary to form a film of 0.2 to 0.3 mil thick in order to obtainseveral hundred hours of protection under extreme or severe conditions.Since half or more of the film thickness is below the initial metalsurface, there will be very little dimensional changes in the treateduranium article over the article prior to treatment.

As various changes may be made in the theory of the protective filmformation, film characteristics, and arrangement of the method stepsherein without departing from the spirit and scope of the invention andwithout sacrificing any of its advantages, it is to be understood thatall matter herein is to be interpreted as illustrative and not in alimiting sense.

What is claimed is:

1. The method of providing a metallic uranium article with acorrosion-resistant surface of uranium oxide, comprising the steps ofremoving contaminants from the surface of the uranium article, confiningthe article in an enclosed volume, evacuating the volume to a pressuresubstantially less than atmospheric pressure, heating the uraniumarticle to a temperature greater than about 500 C. but less than thetemperature at which the uranium changes to beta phase, contacting theuranium article with oxygen at least virtually free of moisture toproduce a uranium oxide film on the surface of the article, introducingthe oxygen into the volume after heating the uranium article to thefirst-mentioned temperature, and maintaining the article a about saidfirstmentioned temperature in the presence of the oxygen for a durationsufficient to provide the film with a thickness in the range of about0.2 to about 1.0 mil, said film being characterized by beingnon-wettable by water, by being virtually free of reaction with andimpervious to dry air, moist air, and water.

2. The method claimed in claim 1, wherein the rranium oxide filmconsists essentially of uranium dioxide and lesser oxides with agradually decreasing gradient in the oxygen content in the film from theoutermost surface thereof inwardly towards the interface between thefilm and metallic uranium, and wherein the uranium oxide adjacent saidinterface consists essentially of uranium monoxide.

3. The method claimed in claim 1, wherein the step of removingcontaminants from the surface of the uranium article includes immersingthe latter in an acid bath for removing naturally occurring oxides, theuranium article is transferred into the enclosed volume after theremoval of the contaminants therefrom and the enclosed volume isevacuated within durations sufiicient to inhibit the formation of excessoxide on the uranium article prior to the heating thereof, and whereinthe pressure substantially less than atmospheric pressure is of apressure in a range corresponding to about 1 to 10 microns.

4. The method claimed in claim 1, wherein the heating of the uraniumarticle is achieved at a rate sufficient to outgas the article prior tothe oxygen contact, and wherein the temperature greater than 500 C. isabout 625 C.

5. The method claimed in claim 1, wherein the oxygen contacting theuranium article is of a quantity sufiicient to effect the reaction U+UO2UO to provide the uranium oxide film with a thickness in said rangetogether with said characteristics.

6. The method claimed in claim 5, wherein the quantity of oxygencontacting the uranium article is less than the quantity sufiicient toeffect the reaction U+O UO 7. The method claimed in claim 6, wherein theoxygen is introduced into the volume at a rate sufficient to provideabout 900 to about 2700 microns oxygen per cubic foot per hour for abouteach 7 square inches of surface area on the uranium article.

8. The method claimed in claim 7, wherein the oxygen is approximately99.9 percent pure.

9. A new article of manufacture comprising a metallic uranium substrateand a corrosion-resinstant uranium oxide film on exposed surfaces ofsaid substrate consisting es sentially of uranium dioxide and lesseroxide and characterized by being non-wettable by water and by having adecreasing gradient in oxygen content from an outermost surface of thefilm inwardly towards the interface between the film and the surface ofsaid substrate.

10. The article of manufacture claimed in claim 9, wherein the film isof a thickness in a range of about 0.2 to 1.0 mil, and wherein theuranium dioxide has a predominant crystal orientation in the 110 planeparallel to the surface of said substrate.

References Cited FOREIGN PATENTS 858,656 1/1961 Great Britain 148-6.3

RALPH S. KENDALL, Primaly Examiner U.S. Cl. X.R. l4831.5

